Prior to the present invention, a satisfactory prosthetic device having a thin continuous film of biocompatible carbon firmly adherent to a substrate has not been generally available. Existing carbon films have not exhibited adherence to their substrate or have not been in a form which is biocompatible. Another deficiency of prior art carbon coated prosthetic devices is that the mechanical properties of the substrates have been adversely affected by the methods and apparatus employed to form the films thereon.
Representative of the prior art techniques for coating a carbon film on a substrate of a prosthetic device include chemical vapor deposition, thermal evaporation by means of an electron beam and ion plating.
In early chemical vapor deposition techniques for forming prosthetic devices, a source for carbon was decomposed at relatively high temperatures and the carbon was deposited in thick layers (e.g., at least 25 microns) on a substrate (see, for example, U.S. Pat. Nos. 3,526,005; 3,685,059 and 3,707,006). Since these techniques typically employed high temperatures e.g., in excess of 1000.degree. C., it was necessary to select substrate materials which were not adversely affected by the high temperatures employed during the process. As a result, the substrate materials employed for the making of prosthetic devices were limited. To this day, such high temperature techniques cannot take advantage of the desirable properties of available low temperature substrate materials.
Efforts to overcome drawbacks attendant to high temperatures have included initially coating the substrates with high temperature organic polymers and then depositing the carbon thereon (see, for example, U.S. Pat. No. 3,952,334). Such techniques are relatively complex and have not met with significant success.
The described prior art techniques also have been relatively costly and have produced a layer of carbon which has not satisfactorily adhered to the substrate. In use it has been found that the carbon layer tends to separate from the substrate and allows body fluids and tissue to attack and degrade the prosthetic device.
Attempts to modify existing chemical vaporization techniques continue. Recently, it has been suggested that catalysts be used to lower processing temperatures. However, such suggestions add to the complexity and costs of the process and result in films which, in use, have experienced undesirable degradation and separation from the substrates intended to be protected.
Other techniques which have been tried and which have not met with success include electron beam evaporation and ion plating of carbon on a substrate. It has been found that the infrared radiation generated by the electron beam evaporation technique causes degradation of the substrate. In addition, the pressures used are so low that the carbon atoms tend to flow in a very straight line and do not uniformly coat a substrate which normally has a morphologically complex surface.
Further, with the electron beam technique, the deposited carbon is not biocompatible and does not firmly adhere to the substrate. The carbon on the substrate is in form of graphite which has been found not to be biocompatible. Also, the deposited carbon tends to separate and is removed from the substrate during use causing body fluids and tissue to attack and degrade the prosthetic device.
With respect to the ion plating technique, it is a modification of vacuum evaporation in which the substrate is held during the deposition at a high negative potential so that it is bombarded by positive ions, in order to improve the adhesion and the structural properties of the film. Obviously this technique cannot be advantageously employed with electrically insulated substrates, like polymers. In fact, it is difficult to electrically bias non-conductive desirable substrates, such as DACRON (a polyester resin), TEFLON (polytetrafluoroethylene) and other relatively low melting polymers.
Thus, available prosthetic devices and techniques are beset by drawbacks. There is a need for prosthetic devices which have a thin biocompatible carbon coating firmly adherent to the substrate, such as low temperature DACRON and TEFLON, etc., and which are relatively simple and inexpensive to produce.